The present disclosure generally relates to appliances, and more particularly to a water heater storage tank that contains a phase change material.
The typical water heater generally comprises an electric resistance element as the primary heater. Such systems tend to consume large amounts of energy and are not energy efficient. As an alternative to less efficient water heating systems “hybrid” heating systems have been introduced. A hybrid water heater generally refers to a heat pump that includes an additional heating element as an auxiliary heater. The term “hybrid” generally refers to any number of heating configurations, such as gas electric or heat pumps. As an example, a “hybrid” system could include an electric heat pump that utilizes relatively low amounts of power, such as for example approximately 1500 watts, that is supplemented with, or uses an electric resistance element as an auxiliary heater. Such an electric resistance element can utilize approximately 3000-4500 watts. Heat pump water heaters produce heat very efficiently, but very slowly. A typical heat pump water heater might move approximately two watts of heat for every one watt of electrical power when heating water slowly. When the demand for hot water increases, the heat pump cannot meet the high heating demand and the heat pump is supplemented by resistive heat, which generally only produces approximately one watt of heat for every watt of electrical power used. To avoid the need for resistance heat, a larger water tank can be used to store additional hot water, but this increases cost and requires more space than is available in many houses.
Currently, when a consumer runs out of hot water, one option is to turn up the water temperature. A higher temperature may require a lower percentage of hot water when taking a shower or bath, and thus reduce the hot water demand. However, the increase in the hot water temperature setting will increase energy use and cost.
Storing heat in a phase change material (“PCM”) can allow a water heater to produce a higher percentage of the heat using the heat pump, because more heat is stored and is ready for high demand periods. A phase change material is a material that is in a solid phase at low temperatures and a liquid phase at higher temperatures. An exemplary phase change material is PureTemp™, developed by Entropy Solutions of Minneapolis, Minn. As the phase change material is heated, its temperature increases until it reaches its melting temperature. At its melting temperature, the phase change material remains in the solid phase while it absorbs a fixed amount of heat, generally understood as the “latent heat of fusion.” Once the phase change material absorbs the fixed amount of heat, the phase change material changes phase from solid to liquid. As heat is removed from the phase change material, its temperature decreases until the phase change material's melting temperature is reached. The phase change material remains in the liquid phase until it releases an amount of heat equal to the latent heat of fusion. As the phase change material continues to lose heat, it changes from the liquid to solid phase. The phase change material can store relatively large amounts of heat without having to be heated to high temperatures. The storage of heat in a water heater or storage unit using a phase change material allows the size of the water heater to be reduced, which reduces heat loss and promotes energy savings.
However, in a situation such as a water storage unit, the phase change material needs to be held in containers that provide a large heat transfer surface area and a large heat transfer coefficient between the container and the water. The arrangement of the phase change material within the cylinder is important in order to prevent the water from bypassing the phase change material, which would reduce the ability and effectiveness of the phase change material to heat the water in the water storage unit.
Low energy use is an important attribute in the design and purchase of appliances. A small exterior volume is important in the purchase of a water heater. Many water heaters are stored in closets with limited volume. A smaller volume or sized water heater will allow the heater to fit in a larger percentage of houses. It would be advantageous to effectively transfer heat from a phase change material to the water and increase the energy storage density for a given size tank or footprint, and water storage temperature.
A potential problem in using phase change material in a water heater is its combination with the resistance or resistive element that is used to heat the water in a water heater. If the resistive element comes in contact with the phase change material, or the container therefor, damage can occur. Thus, the phase change material in a water heater storage tank needs to be kept away from the resistive heater to prevent damage to the phase change material and heater. It is also necessary to prevent damage to the phase change material in the event of a water leak.
Accordingly, it would be desirable to provide a system that addresses at least some of the problems identified above.